The present invention pertains to a symbol encoding method and apparatus optimized for energy efficient transmission.
Coding and modulation refers to the mapping of information into a representation, or code, suitable for transmission on a channel. Measures for rating the performance of a coding protocol include: transmission rate, error probability, and power consumption. Since these criteria exhibit conflicting objectives, their application results in performance tradeoffs. Communications systems typically aim to maximize transmission rate, while keeping error probability and power consumption below specified thresholds. For wireless communication applications, however, minimization of power consumption is highly desirable in order to extend battery lifetimes. And, indeed, the overall power consumption of a wireless communication system is driven by transmitter operation which is, in turn, controlled by the communication protocol.
In accordance with one aspect of the invention, a method for telemetering data on a channel having a maximum data rate by utilizing a power efficient communication protocol is provided. The data to be telemetered comprises a number of source symbols where each source symbol is characterized by a probability of occurrence. A sequence of signals forming codewords having an energy and a length is produced. The length for the codewords is based at least on the number of symbols to be coded. Each codeword signifies a specified source symbol on the basis of a mapping between source symbols and codewords. The mapping is such that each source symbol of lower probability of occurrence is associated with a codeword of at least equal energy. In a related embodiment, the codeword length is also determined based on the maximum data rate of the channel. In another related embodiment the codewords and source symbols may be concatenated to provide an increase in power efficiency.
In yet another embodiment of the invention, the power efficent communication protocol may be combined with other coding schemes. The codewords are first constructed based upon the chosen coding scheme. Examples of coding schemes are turbo coding, trellis coding, lattice coding, error correction coding, multi-level modulation and block-coded modulation. The length of the codewords is constrained by the number of symbols and a given transmission rate for the channel. A mapping is then producing between the source symbols and codewords mapped such that each symbol of lower probability of occurrence is associated with a codeword of at least equal energy.
In another embodiment of the invention an apparatus for implementing the power efficient communication protocol is provided. The apparatus includes an encoder for converting source symbols to codewords on the basis of a mapping between source symbols and codewords. The length of the codewords is determined in part on the number of source symbols and the number of source symbols per second to be transmitted. The mapping between the source symbols and the codewords is such that each symbol of lower probability of occurrence is associated with a codeword of at least equal energy. The apparatus further includes a transmitter for transmitting the codewords. The transmitter transmits the codewords imposed on a carrier wave to a receiver which demodulating the codewords provides the codewords to a computer or processor. The computer/processor is equipped to determine the source symbols corresponding to the codewords.